A woman in the United States was accidentally locked overnight in a prison visiting area while attempting to visit an inmate.

The incident took place on Oct 4 when the unnamed woman arrived at the maximum-security Theo Lacy detention facility in Orange County in California, before visiting hours ended at 5pm.

US news reports said the prisoner she wanted to see was unavailable, so the woman, who was in her 30s, was directed by a prison employee to the public visiting area.

She fell asleep while waiting, and was locked inside. No one realised she was there until the next morning, when employees returned to work at 8am.

They freed her and treated her for a cut in her hand. It is unknown how or when she sustained the injury, said CBS News.

Orange County Sheriff’s Department spokeswoman Carrie Braun said state laws mandated that no cell phones can be taken inside the compound, so the visitor was unable to make any calls when she was locked inside alone.

There are no public phones in the waiting area.

Local newspaper The Orange County Register said security cameras inside the waiting area can be monitored only from the guard station inside the room, which was not manned overnight.

Ms Braun said the woman was not in any immediate danger as she was in an area not accessible by prisoners.

Still, the sheriff department said it has launched a probe.

“This unfortunate incident should never have occurred,” Ms Braun said. “The department is committed to fully investigate and ensure this never happens again.”

At the same time, Ms Braun said an emergency phone will be installed in the area, while a supervisor will make periodic checks overnight.